This invention relates in general to xerography and, more specifically, to a novel photoconductive device and method of use.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the nonilluminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated by U.S. Pat. No. 3,121,006 which describes a number of layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In its present commercial form, the binder layer contains particles of zinc oxide uniformly dispersed in a resin binder and coated on a paper backing.
In the particular examples described in '006, the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductor particles for any significant distance. As a result, with the particular material disclosed, the photoconductor particles must be, in substantially continuous particle-to-particle contact throughout the layer in order to permit the charge dissipation required for cyclic operation. Therefore, with the uniform dispersion of photoconductor particles described, a relatively high volume cencentration of photoconductor, about 50 percent by volume, is usually necessary in order to obtain sufficient photoconductor particle-to-particle contact for rapid discharge. However, it has been found that high photoconductor loadings in the binder results in the physical continuity of the resin being destroyed, thereby significantly reducing the mechanical properties of the binder layer. Systems with high photoconductor loadings are often characterized as having little or no flexibility. On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the photoinduced discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.
U.S. Pat. No. 3,037,861 to Hoegl et al teaches that poly(N-vinylcarbazole) exhibits some long-wave length U.V. sensitivity and suggests that its spectral sensitivity can be extended into the visible spectrum by the addition of dye sensitizers. The Hoegl et al patent further suggests that other additives such as zinc oxide and titanium dioxide may also be used in conjunction with poly(N-vinylcarbazole). In the Hoegl et al patent, the poly(N-vinylcarbazole) is intended to be used as a photoconductor, with or without additive materials which extend its spectral sensitivity.
In addition to the above, certain specialized layered structures particularly designed for reflex imaging have been proposed. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two-layered zinc oxide binder structure for reflex imaging. The Hoesterey patent utilizes two separate contiguous photoconductive layers having different spectral sensitivites in order to carry out a particular reflex imaging sequence. The Hoesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.
It can be seen from a review of the conventional composite photoconductive layers cited above, that upon exposure to light, photoconductivity in the layered structure is accomplished by charge transport through the bulk of the photoconductive layer, as in the case of vitreous selenium (and other homogeneous layered modifications). In devices employing photoconductive binder structures which include inactive electrically insulating resins such as those described in the U.S. Pat. No. 3,121,006, conductivity or charge transport is accomplished through high loadings of the photoconductive pigment and allowing particle-to-particle contact of the photoconductive particles. In the case of photoconductive particles dispersed in a photoconductive matrix, such as illustrated by U.S. Pat. No. 3,121,007, photoconductivity occurs through the generation and transport of charge carriers in both the photoconductive matrix and the photoconductor pigment particles.
Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of repetitive xerographic cycling where these photoconductive layers are susceptible to abrasion, chemical attack, heat and multiple exposure to light. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.
In addition to the problems noted above, these photoreceptors require that the photoconductor comprise eith a hundred percent of the layer, as in the case of the vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of a photoconductive layer containing all or a major proportion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are primarily dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.
Another form of a composite photosensitive layer which has also been considered by the prior art includes a layer of photoconductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate.
U.S. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlies a layer of vitreous selenium which is contained on a supporting substrate. In operation, the free surface of the transparent plastic is electrostatically charged to a given polarity. The device is then exposed to activating radiation which generates a hole electron pair in the photoconductive layer. The electrons move through the plastic layer and neutralize positive charges on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic materials which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.
U.S. Pat. No. 3,598,582 to Herrick et al describes a special purpose composite photosensitive device adapted for reflex exposure by polarized light. One embodiment which employs a layer of dichroic organic photoconductive particles arrayed in oriented fashion on a supporting substrate and a layer of poly(N-vinylcarbazole) formed over the oriented layer of dichroic material. When charged and exposed to light polarized perpendicular to the orientation of the dichroic layer, the oriented dichroic layer and poly(N-vinylcarbazole) layer are both substantially transparent to the initial exposure light. When the polarized light hits the white background of the document being copied, the light is depolarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion throughout the layer of poly(N-vinylcarbazole).
Belgium Pat. No. 763,540, issued Aug. 26, 1971, discloses an electrophotographic member having at least two electrically operative layers. The first layer comprises a photoconductive layer which is capable of photogenerating charge carriers and injecting the photogenerated holes into a contiguous active layer. The active layer comprises a transparent organic material which is substantially nonabsorbing in the spectral region of intended use, but which is "active in that it allows injection of photogenerated holes from the photoconductive layer, and allows these holes to be transported through the active layer. The active polymers may be mixed with inactive polymers or nonpolymeric material.
Gilman, Defensive Publication of Ser. No. 93,449, filed Nov. 27, 1970, published in 888 O.G. 707 on July 20, 1970, Defensive Publication No. P888.013, U.S. Cl. 96/1.5, discloses that the speed of an inorganic photoconductor such as amorphous selenium can be improved by including an organic photoconductor in the electrophotographic element. For example, an insulating resin binder may have TiO.sub.2 dispersed therein or it may be a layer of amorphous selenium. This layer is overcoated with a layer of electrically insulating binder resin having an organic photoconductor such as 4,4'-diethylamino-2,2'-dimethyltriphenylmethane dispersed therein.
"Multi-Active Photoconductive Element", Martin A. Berwick, Charles J. Fox and William A. Light, Research Disclosure, Vol. 133; pages 38-43, May 1975, was published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire, England. This disclosure relates to a photoconductive element having at least two layers comprising an organic photoconductor containing a charge transport layer in electrical contact with an aggregate charge generation layer. Both the charge generation layer and the charge transport layer are essentially organic compositions. The charge generation layer contains a continuous, electrically insulating polymer phase and a discontinuous phase comprising a finely divided, particulate cocrystalline complex of (1) at least one polymer having an alkylidene diarylene group in a recurring unit and (2) at least one pyrylium-type dye salt. The charge transport layer is an organic material which is capable of accepting and transporting injected charge carriers from the charge generation layer. This layer may comprise an insulating resinous material having 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane dispersed therein.
U.S. Pat. No. 3,265,496, discloses that N,N,N',N'-tetraphenylbenzidine may be used as photoconductive material in electrophotographic elements. This compound is not sufficiently soluble in the resin binders of the instant invention to permit a sufficient rate of photoinduced discharge.
Straughan, U.S. Pat. No. 3,312,548, in pertinent part, discloses a xerographic plate having a photoconductive insulating layer comprising a composition of selenium, arsenic and a halogen. The halogen may be present in amounts from about 10 to 10,000 parts per million. This patent further discloses a xerographic plate having a support, a layer of selenium and an overlayer of a photoconductive material comprising a mixture of vitreous selenium, arsenic and a halogen.
In the context of a dual layer system, i.e. a charge generating layer and a charge transporting layer, most organic charge transporting layers using active materials dispersed in organic binder materials have been found to trap charge carriers causing an unacceptable buildup of residual potential when used in a cyclic mode in electrophotography. Also, most organic charge transporting materials known when used in a layered configuration contiguous to a charge generating layer have been found to trap charge at the interface between the two layers. This results in lowering the potential differences between the illuminated and nonilluminated regions when these structures are exposed to an image. This, in turn, lowers the print density of the end product, i.e. the electrophotographic copy.
Another consideration which is necessary in the system is the glass transition temperature (T.sub.g). The (T.sub.g) of the transport layer has to be substantially higher than the normal operating temperatures. Many organic charge transporting layers using active materials dispersed in organic binder material have unacceptably low (T.sub.g) at loadings of the active material in the organic binder material which is required for efficient charge transport. This results in the softening of the layer, which in turn, may become susceptible to impaction of dry developers and toners. Another unacceptable feature of a low (T.sub.g) is the case of leaching or exudation of the active materials from the organic binder material resulting in degradation of charge transport properties from the charge transport layer. Another deficiency of the low (T.sub.g) layers is the susceptibility to crystallization resulting from increased diffusion rates of the small molecules.
Another consideration for the use of organic transport layers in electrophotography is the value of the charge carriers mobilities. Most of the organics known to date are deficient in this respect in that they set a limit to the cyclic speed of the system employing the same. None of the references above overcome the foregoing problems.